DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY
Human T lymphocytes (pre-T cells) and activated platelet, scanning electron micrograph (SEM). Lymphocytes are involved in the specific immune response. Pre-T cells circulate in the blood before migrating to the thymus where they develop into helper T cells or killer T cells that are able to identify antigens and infected cells. Platelets are blood cell fragments that play an essential role in blood clotting and wound repair. Platelets in the blood are small oval disks and are termed non-activated. When an injury such as a cut is sustained, platelets become activated, change their shape, become sticky and build up on a vessel wall to form a plug. They are also involved in the secretion of a chemical platelet factor which helps produce threads of fibrin. Bloods cells collect in the entangled fibrin and platelet mass forming a blood clot at the site. Magnification: x2,200 when shortest axis printed at 25 millimetres.
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